Conventional scraped surface heat exchangers are utilized in various food processing applications. For example, equipment of this general type may include refrigeration systems or other systems having a heat exchange surface used in the continuous production of food product such as ice cream, other frozen desserts, frostings and crystallized oils. For example, the refrigeration system used for production of frozen desserts typically comprises an elongated freezing cylinder with an interior heat exchange surface and a dasher assembly disposed with attached blades disposed proximate to the heat exchange surface. The dasher assembly is rotated by a dasher motor drive so that the attached blades scrape ice crystals off the interior wall of the freezing cylinder. In the production of other food products, sugar crystals or fat crystals are scraped from the heat exchange surface.
In order to process frozen food product, the scraped surface heat exchanger receives unfrozen product mix from an inlet section and provides processed product through an outlet section to a filler. Typically, the freezing cylinder is initially filled with unfrozen product mix. Next, the refrigeration system surrounding the freezing cylinder is started. Once the freezing cylinder is filled and the refrigeration process has begun, the dasher assembly rotated in order to both mix the product and to introduce ice crystals that are formed on the freezing cylinder wall into the product mix.
The decreasing temperature and the introduction of the ice crystals both act to increase the viscosity of the product mix. Accordingly, the equipment utilized must have sufficient strength to withstand relatively high torque forces caused when the flowing product exceeds a certain amount. In addition, the equipment must be particularly durable to accommodate the scraping of ice crystals formed on the heat exchange surface.
For this reason, scraping blades attached to the outer periphery of the dasher assembly in known arrangements are fabricated of metal such as stainless steel. Although the use of plastic blades would be desirable since they offer a cost advantage over metal blades, plastic blades cannot be used to engage the heat exchange surface for scraping applications such as where ice crystals are formed on the inside surface of the freezing cylinder. In addition, the use of known plastic blade arrangements would involve health risks when used in these types of applications. This is principally due to the difficulties in detecting the presence of such materials when they are accidentally introduced into the food product.
The use of metal blades, however, also poses several shortcomings in known designs. As mentioned earlier, metal blades are rather costly and often difficult to fabricate. In addition, metal blade arrangements in known designs must frequently be resharpened in order to be effective in operation. This is particularly true in applications for producing frozen product since the blades continuously scrape the ice crystals off the inside of the freezing cylinder. Therefore, the production process must be frequently interrupted so that the metal blades can be removed, then resharpened either by hand or by machine, and then reinserted into the dasher assembly. This results in higher maintenance costs and increased production down time.